Charge pump voltage generators make it possible to produce a direct current (DC) electrical voltage higher than a specified supply voltage. In the field of integrated circuits, charge pumps may be used for producing, for example, the high erase and programming voltage VPP for the floating grid transistors of electrically erasable and programmable memories (EEPROM, FLASH, FLASH-EEPROM, etc.).
A charge pump is conventionally controlled by clock signals of opposite phase supplied by an oscillator. The charge pump may include a plurality of cascaded pump stages whose structure may be well known to those skilled in the art. The charge pump receives as an input, a supply voltage VC of 2 to 5 volts. The amplitude of the output voltage VPP depends on the total number of pump stages in cascade and is proportional to the supply voltage VC.
Typically, to program or to erase an EEPROM memory, the charge pump voltage generator must be able to supply a voltage of 15 to 18 V with a current of some tens of microamps. If the voltage applied to the memory is too small, the memory cells programmed or erased are in an uncertain state. The result of this is that the memory will not be functional. On the contrary, if the voltage applied to the memory is too high, the transistors of the memory cells and of the high voltage stage initially undergo a stress which reduces the operational lifetime of the memory (aging of the transistors, breakdown of the grid oxides of the floating grid transistors, etc.). For these reasons, it is recommended to limit the variations of the high voltage applied to the memory to about 1 V. These variations take account of the variations in temperature, of the supply voltage, of the output consumption of the charge pump, of the manufacturing technology of the memory, of the frequency of the oscillator, and of the presence of stray resistances or capacitances, etc.
The charge pump of a voltage generator is generally provided with a number of pump stages greater than the theoretically sufficient number of stages to be able to support a large range of operating voltages and to compensate for the high internal impedance of the charge pump. Because of this, after a start-up period, the charge pump can deliver a voltage VPP greater than the threshold VPPmax beyond which transistors to be erased or programmed could be damaged. On the other hand, the supply voltage VC can fluctuate strongly (plus or minus 10%) with respect to its nominal value taken into account during the design of the charge pump, and an increase in the voltage VC can give rise to a corresponding increase in the voltage VPP beyond the threshold VPPmax. Control of the voltage VPP is provided to not exceed the threshold VPPmax. This control can be provided by a voltage regulation circuit.
Such a regulation circuit can, for example, control the frequency of the oscillator controlling the charge pump. The oscillator can thus be a voltage controlled oscillator, and the regulation circuit then delivers a control voltage to the input of the oscillator. However, a voltage controlled oscillator has a small frequency range, and it is difficult to design such an oscillator which is robust with respect to variations in temperature or load current. Moreover, such an approach also exhibits variable frequency interference noise.
It is also possible to provide a regulation circuit which stops or activates the voltage generator depending on whether the voltage to be regulated is above or below a set voltage. More precisely, the circuit provides a control signal to an activation input of the oscillator of the charge pump according to the comparison between the output voltage VPP and a reference voltage. However, the regulation carried out by such a circuit has a relatively slow instantaneous response. In particular, it does not make it possible to absorb a transient overvoltage at the output of the charge pump. An overvoltage at the level of the supply voltage VC can give rise to a sudden increase in the amplitude of one of the clock signals. There is a risk that the sudden increase will be instantaneously reflected, by the charge pump, in the form of an overvoltage on the voltage VPP, principally because the reaction of the charge pump to a stop command is not instantaneous. The result of this is that the overvoltage, which thus appears in the voltage VPP, exceeds the reference voltage before the charge pump is stopped, and can therefore damage the dipole fed by the charge pump. Moreover, such an approach also exhibits variable frequency noise interference.
Finally, it is also possible to provide a regulation circuit which controls the efficiency of the oscillator to control the transfer of charges. However, by reducing the efficiency of the oscillator, the efficiency of the circuit is also reduced, which involves energy losses. Moreover, the frequency response of the charge pump is not linear, which causes stability difficulties in the circuit when a control loop is used.